The 55 1.4 seems to be a pretty amazing lens. There are two types of commenters on this lens, those who don't own it and see lot's of problems, those who own it and love it. Beware of taking advice from those who don't own the lens.
Hard to believe that people are still talking about SDM failure. That is so 6 years ago. That would be like me saying "don't buy a Nikon, look at the sensor stand problem." Pentax's SDM problems are older than the Nikon stain problem, but in the minds of some Nikon users, the stain problem that got Nikon banned in both Russia and China should be forgotten. Pentax's gaffes should be remembered forever. All Pentax pro motors are called SDM . There is no relationship between the original SDM and the motors now called SDM.
As for lens availability, many of us have FA lenses, there are millions of them, and they work just fine. Too funny saying there are only 12 lenses available for it. I myself own more than 20.
As for lens quality, if that's an issue I'd currently go with the Sony, however Pentax is within months of releasing a new 50 1.4 which will be as modern as anything out there. It will be expensive but so is the Sony. I bought my K-1 a year ago, and it's an incredible camera. For still life using Pixel Shift it produces superior images to a D850, especially at high ISO, but slightly lower IQ in normal mode. The big issue for me is the price of Sony systems. I simply don't have that kind of money. With a K-1 and a few ancient lens I can compete with the best and often do. I'd get a K-1 and go for the new FDA 50 1.4. Copies of the lens are in the hands for the lens testers and sample images are being posted. It's less than 2 months away from production. An older FA 50 1.4 might keep you happy until then if you don't want the expense of the 55 1.4.
So in the end, my advice, if you want it today, go for the Sony, if you can wait 4 months, consider the new Pentax 50 1.4 to be released this spring and K-1. The hit rate is fine, I regularly shoot beside Canon, Nikon and Sony shooters. if there is a hit rate difference it's minor. As one comparative review comparing the Pentax, Nikon, Sony and Pentax, the Pentax is s little slower, you are talking hundredths of a second slower. Unless you are shooting high speed sports or birds in flight Pentax can keep up with anyone. One BiF shooter told me his keeper rate went up to 75% from about 40% switching to Nikon, but I often shoot 500 frames to get 5 or 6, so 200 images to choose from instead of 350, I can live with that. I get what I need, and avoid the extra expense.
And if you really want 55mm, give the {metas 55 1.4 is great lens. It might work for you.